Design of Heat Exchangers

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Design of Heat Exchangers

• Dick Hawrelak
• Presented to CBE 497 on 31 Oct 00 at UWO

2
Introduction

• Design using HTRI and based on TEMA Stds
• TEMA Shell Head Types, Perry VI, page 11-4
• TEMA nomenclature, Perry VI, page 11-6
• Liquid / liquid exchanger design example
• RW Condenser example on CD-ROM

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TEMA BEM Exchanger
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Plant Design, (11) Exchangers

• Heat Recovery Efficiency
• Colburn heat transfer method for hi
• CLMTD Correction Factor, Perry VI, p-10-27
• Heat Exchanger Materials
• Liquid Liquid Exchanger design example
• RW Condenser design example
• Shell Size V1.1 for kettle shell diameter
• Tube Count Exchanger Comparison

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Approximate Design MethodTube Count Exchanger
Comparison
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Quick Approximate Method

• Assume Design Ud values, Perry VI, p-10-44.
• BTU/hr temperatures from process simulation
• Assume heating or cooling temperatures
• Calc LMTD, correct to CLMTD, if required
• Calc Area Q / Ud / CLMTD

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Approx Method Continued

• Assume tube od, BWG, tube length, to calc no.
  tubes (Table 11-2)
• Assume no. tube passes. Determine shell diameter,
  Perry VI, Table 11-3 tube count
• Assume materials get cost estimate for
  exchanger

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Pressure Drop

• Exchanger area vs pressure drop.
• Economics often dictate pressure drop.
• The designer sets the allowable pressure drops
  during simulation of process.
• Confirm pressure drops during exchanger design.
• Nozzle sizes, baffle spaces, tube dia., tube
  length, no. tubes per pass all affect pressure
  drop.

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Fouling and Overdesign

• Fouling factors are specified to give the
  exchanger a cleaning cycle (eg 1 year).
• In clean hydrocarbon services, a dirt factor of
  0.001 is specified on both sides.
• The combination of heat transfer coefficients,
  fouling and material resistance allow calculation
  of a clean heat transfer coefficient, Uc

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Over-design Problems

• Exchanger is designed with a Ud and a
  corresponding fouled CLMTD.
• On start-up, the exchanger operates with a Uc and
  a clean CLMTD. This may result in flow problems
  for condensing systems.
• Which steam pressure or refrigerant level should
  be used?

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Temperature Profiles

• Manual calculations use average in out
  temperatures.
• Subcooling affects LMTD.
• Partial condenser temperature profiles with inert
  gases are difficult to model.
• Good VLE data hard to obtain.

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Mechanical Design

• High RHO-V-SQUARE on inlet shell nozzle can
  rupture tubes.
• Impingement plate design not well defined.
• Tube vibrations with long tube spans.
• How to join tubes to tubesheet?

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Maldistribution

• Shell side maldistribution with small window
  cuts. Use 20 baffle cuts.
• Tube side maldistribution with low tube side
  pressure drops. Long tubes, small tube diameters.
• Chinese hat diffusers on tube and shell sides.

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Acoustics

• Shell side geometry can cause acoustic
  vibrations.
• May require tuning baffles.

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Entrainment

• Minimize entrainment in Kettle refrigeration
  coolers. See Shell Size V1.2.
• Entrainment levels often ignored on mass
  balances.
• Kettle vapor outlets flow to KO pots in
  refrigeration compressor design.

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Expansion Joints.

• Expansion joints when shell and tubes are
  different materials.
• Expansion joints are a hazard.
• Expansion joints are fragile.
• No. flexes per hour usually unknown.
• Paper clip example.

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Reboiler Recirculation Problems

• Low Recirculation due to inert build-up in shell,
  high tube resistance, low liquid level in column.
• Low recirculation promotes fouling and unwanted
  heavies production.
• Seadrift EO tower explosion due to faulty
  reboiler design,

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Thermosyphon Layout
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Design of Heat Exchangers

• Method by Lord, Minton and Slusser, of UCC
• 26 Jan 70, Chemical Engineering, p-96.
• Methods suitable for all types of exchangers.
• Method suitable for spreadsheet analysis.
• See Liquid Liquid Exchanger and
  RW Condenser in Plant Design, Exchangers.
• Alternatively, Process Heat Transfer by Kern

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Input Data
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Heat Balances

• Tubeside (Wi)(ci)(tH tL) (hi)(A)(dTi)
• Tube walls ((Wi)(ci)(tH tL) (hw)(A)(dTw)
• Fouling (Wi)(co)(tH tL) (hs)(A)(dTs)
• Shellside (Wo)(co)(TH TL) (ho)(A)(dTo)
• dTi dTw dTs dTo LMTD dTM
• dTi/dTM dTw/dTM dTs/dTM dTo/dTM 1

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Heat Balances Continued

• Tubeside (Wi)(ci)(tH tL) / (hi)(A)(dTM)
• Tube walls ((Wi)(ci)(tH tL) / (hw)(A)(dTM)
  
• Fouling (Wi)(co)(tH tL) / (hs)(A)(dTM)
• Shellside (Wo)(co)(TH TL) / (ho)(A)(dTM)
• 1.0

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Heat Transfer Coefficients

• hi 0.023ciGi/(ciui/ki)0.67/(DiGi/ui)0.2
• hw 24kw / (do di)
• ho 0.33coGo(0.6)/(couo/ko)0.67/(DoGo/ko)0.2
• hs assumed value

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Arrange Equations Into 4 Factors

• For example for dTi/dTM for inside tubes,
  no phase change, liquid, Nre gt 10,000
• Numerical factor, f1 10.43
• Physical Property Factor
  f2 (Zi0.467Mi0.22)/si0.89
• Work factor f3 Wi0.2(tH tL) / dTM
• Mechanical Design Factor, f4 di0.8/n0.2/L
• dTi / dTM (f1)(f2)(f3)(f4)
• Similarly for hw, ho and hs

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Pressure Drops

• Tubeside pressure drop, psi, Eqn (21)
  DP (Zi0.2/si)(Wi/1000/n)
  1.8((L/di)25)/(5.4di)3.8
• Shellside pressure drop, psi, Eqn (25)
  DPs (0.326)/So(Wo/1000)2
  (L)/Ps3/Ds

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Step 1 Calculate Heat Duty
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Step 2
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Step 3
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Step 4
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Step 5
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Step 6
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Step 7
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Step 8 Heat Transfer Calcs
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Step 8 Continued
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Step 8 Continued

• dTi/dTM (f1)(f2)(f3)(f4)
• dTi/dTM (10.43)(4.27)(0.62)(0.0193) 0.5339
• Similar Calculations for tube wall, fouling and
  shell side.

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Sum of Products Summary
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End of Presentation

• Good luck on your exchanger designs.
• If you have any questions call
• rhawrela_at_xcelco.on.ca